Signal processing apparatus for multiplex transmission



XR 3548107 SR James E. Webb Administrator of the National Aeronauticsand Space Administration with Respect to an Invention of;

Lawrence Y. Lim, Monterey Park, Calif. Appl. No. 725,432

Filed April 30, 1968 Patented Dec. 15, I970 Inventors SIGNAL PROCESSINGAPPARATUS FOR MULTIPLEX TRANSMISSION 7 Claims, 4 Drawing Figs.

324/77 Int. Cl H04j 3/00 Field of Search 179/ IAS,

15.55, 150R, 15A, ISBWR; 324/77 {56] References Cited UNITED STATESPATENTS 3,349,183 10/1967 Campanella H 179/1555 3,384,715 5/1968 Higuchi179/15(OR) 3,435,147 3/1969 Malm 179/15(A) 2,768,352 lO/1956 Von Sivers179/15(ACE) Primary Examiner-Ralph D. Blakeslee Attorneys-J. H. Warden,D. E. Leslie and G. T. McCoy COMMUT.

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I I N I I w I FIG 2 84 cos I 1 as l SINE l l I T 86 l I INVENTUR. W 1LAWRENCE, Y LIM T i V l l i UL/L,/YZI f SIM-1 3 I l gm L ATTORNEYSSIGNAL PROCESSING APPARATUS FOR MULTIPLEX TRANSMISSION ORIGIN OF THEINVENTION The invention described herein-was made in the performance ofwork under a NASA contract and is subject to the provisions of Section305 of the National Aeronautics and Space Act of I958, Public Law 85-568(72 Stat. 435; 42 USC 2457).

BACKGROUND OF THE INVENTION This invention relates to multiplextelemetry system and more particularly to signal processing apparatusfor use in such systems.

Many applications require the use of time multiplexed telemetry systemsfor successively coupling a number of separate information channels to acommon communication channel. For example, such systems are used tomonitor various conditions of a spacecraft such as temperature andvoltage levels. The sensors detecting these conditions are periodicallysampled by a commutatorlike device and the samples are then coupled tothe communication channel. Each sample can represent merely the integralof a sensor output over a sampling time. However, if the monitoredcondition is changing or oscillating rapidly, such information will notbe transmitted, and the rapid variations will go entirely unnoticed. Forexample, if a voltage supply is rapidly oscillating, a simple periodicsample at long intervals will not indicate that such oscillations areoccurring.

One way of detecting oscillations or rapid variations is to take manymeasurements during each sampling period. However. this results in anundue increase in the bandwidth requirements of the transmitter. This isparticularly true if the many measurements are taken and transmittedregardless of whether or not there are rapid variations. A samplingsystem which indicated the level of rapid variations occurring during asampling interval with a minimum of bandwidth, and in a manner whichfacilitated reconstruction of the signal, would be useful incommunication where a limited bandwidth was available. If the samplescould also be used to indicate whether a few or many measurements shouldbe transmitted for any given sampling interval, an even greaterreduction in bandwidth requirements could be achieved.

OBJECTS AND SUMMARY OF THE INVENTION One object of the present inventionis to provide apparatus for generating signal samples having more usefulinformation for a given transmission bandwidth.

Another object is to provide signal samples which enable a more accuratereconstruction of the original signal.

In accordance with the present invention, there is provided apparatusincluding a commutator for periodically sampling a number of channels.Each of the channels which is sampled carries an analogue signal whichmay vary within a wide range of frequencies. A plurality of separatesampling circuits is connected to the output of the commutator. Eachsampling circuit monitors the signal during the sampling time todetermine the amplitude of a particular frequency component. The sum ofthese components can be used to reconstruct the original signal by aFourier approximation. In order to conserve bandwidth. the apparatus mayalso include circuitry for allowing the transmittal of only one or alimited number of low frequency components of the sample during thosesampling intervals when the signal has no high frequency components.

In one embodiment of the invention a commutator switch advances from onesignal source to the next at the end of each sampling period. The outputof the switch is processed by a complex of many Fourier coefficientgenerators. Each of the generators multiplies the input signal by asignal of a predetermined frequency, and integrates the product during asampling period to obtain the Fourier coefficient for that frequency.Generally. a pair of coefficient generators is used for each frequency,one of which multiplies the input signal by a sine wave and the other bya cosine wave (i.e., out of phase with the sine wave wave) of the samefrequency. In addition, the simple integral of the input signal over thesampling period is derived. All of these coefficients may be transmittedat the end of each sampling period to enable reconstruction of the inputsignal.

In another embodiment of the invention, logic circuitry is used totransmit only those coefficients of appreciable magnitude. The fact thatthe magnitude of all coefficients, up to a certain frequency, arealready derived simplifies the determination of which ones are ofappreciable magnitude.

The novel features of the invention are set forth with particularity inthe appended claims. The invention will best be understood from thefollowing description when read in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a block diagramrepresentation of sampling apparatus constructed in accordance with theinvention;

FIG. 2 shows the waveforms of various signals utilized in the circuit ofFIG. 1;

FIG. 3 is a block diagram representation of another embodiment of theinvention; and

FIG. 4 is a block diagram representation of still another embodiment ofthe invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. I shows a signalprocessing circuit for sampling a number of input signals received atfive input channels 11, 12, l3, l4, and 15. These five channels may bethe outputs from various sensors aboard a spacecraft, such as those thatsense temperatures, voltage levels, and radiation levels. The inputsignals are received by a commutator switch 18 having a contact 20 whichis driven at a constant rate to periodically contact each of the fiveinput channels for a predetermined sampling interval. The output of theswitch is delivered at a terminal 22 to a sampling complex 24. Thesampling complex comprises a plurality of coefficient generators 26, 28,30, 32, and 34. Each of the coefficient generators derives the amplitudeof a particular frequency component in the input signal being sampled,during the sampling time.

The coefficient generators 26 through 34 provide coefficients whichenable reconstruction of a sample by a Fourier approximation. It is wellknown that any periodic wave can be closely approximated by the sum of alimited number of sinusoidal functions. A Fourier series approximationis commonly given as:

Equation 1 where A is the average value, A,- and B, are the amplitudesof the various coefficients at the frequencies f i and T equals theperiod of the periodic function. This same analysis can be applied toapproximate a nonperiodic waveform during the period T by applying itonly to that interval. In this case, T can equal one-half the samplingperiod. For a sampling apparatus wherein the sampling period T is alwaysthe same, the only data which must be transmitted are the levels of thecoefficients A and B.

The outputs of the coefficient generators 26 through 34 are thecoefficients of the sinusoidal wave, which approximate the sample duringthe sampling period. The output 36 labeled A is the average value of thesignal during the sampling period. The outputs 38 through 44, labeled AB, through A B indicate the cosine and sine coefficients at variousfrequencies. Knowledge of each of these components allows thereconstruction of the sample, and the greater the number of thesecoefficients, the greater the accuracy of the reconstruction.

The coefficient generator 26 comprises an integrator 46 which integratesthe input signal during the sampling period,

and a hold circuit 48 which holds the value received at the end of thesampling period. The next coefficient generator 28 comprises amultiplying circuit 50 having an input from the sampling terminal 22 andanother input from a cos generator 52 (where T is the samplinginterval). The output from the multiplying circuit 50 is entered into anintegrator 54 which has been reset to zero at the beginning of thesampling interval. The integral of the multiplying generator output forthe sampling period T is equal to the coefficient of the lowestfrequency cosine wave, A,. This value of the integral is held by asecond hold circuit 56 for later readout.

The third coefficient generator 30 includes a multiplying circuit 58which multiplies the input signal at terminal 22 by the signal from agenerator 60 which generates the waveform The product is integratedduring a sampling period by an integrator 62, and the value of theintegral is held by a hold circuit 64. The output of the hold circuit 64is the coefficient B, in the Fourier equation given above as equation 1.

The sampling complex 24 includes additional coefficient generatorsbetween those at 30 and 32, which are similar to the generators 28 and30, but which multiply the input signal by the higher order cosine andsine waves of the Fourier series. These are, of course, integralmultiples ofa fundamental sin frequency, such as the frequency T A lastpair of coefficient generators 32 and 34 includes sinusoidal generators66 and 68 which generate signals equal to the cosine and sine of T mbuen is any integer. The larger the value of n, the more accurately can thewaveform sampling be given, particularly where rapid variations occurduring the sampling period. The output of multipliers 67 and 69 areintegrated by integrators 71 and 73. The output of the hold circuits 70and 72 are the coefficients A,, and B,,.

At the end of a sampling period T, the hold circuits of the samplingcomplex 24 constantly deliver an analogue signal with an amplitude equalto the Fourier coefficients A,,, A,, B,, etc. These outputs aredelivered to a commutator switch 74 where a movable contact 76,controlled by a commutator operating circuit 75, is sequentiallyconnected to each of the lines 36, 38, 40, etc., to 42, and 44. Theoutput 77 of contact 76 is connected to an analog-to-digital converter78, which converts each analogue Fourier coefficient to a digital value.The output of the ADC 78 is delivered to a transmitter circuit 80 forprocessing and transmission.

At the end of each sampling period T, the contact of the commutatorswitch 18 is moved to a next input channel, such as channel 14. At thesame time, the sampling complex 24 must be prepared to sample the newsignal by resetting the value in the integrators 46, 54, 62, etc., tozero. The values in the hold circuits 48, 56, etc., are not changeduntil the end of the new sampling period. During the next period ofduration T, when a new channel is being sampled, the contact 76 ofcommutator switch 74 makes contact with every output A through B,, ofthe sampling complex. At the end of the new sampling period, the valuesin the hold circuits 48, 56, etc., are reset, and the new values fromthe integrators 46, 54, etc., are entered therein. A clock (not shown)is connected to the hold circuits, integrator circuits, and commutatorswitch to reset and operate them.

All of the cosine and sine generators 52, 60, etc., are synchronizedwith each other and synchronized with the sampling periods. Accordingly,all of the of the cosine and sine signal begin from their initial valuesat the beginning of each sampling period, except for the first twosinusoidal generators 52 and 60. The generators 52 and 60 generate theinverse of the cosine and sine signals at every other period, but thisfact can be taken into account at the receiver during reconstructionofthe sample. Alternatively, the output ofthe generators 52 or 60. or ofthe multiplying circuits, integrators, or hold circuits of the first twocoefficient generators can be multiplied by -l at every other cycle. Itshould be understood that waveforms other than sinusoidal waves can beused; for exampie. square waves with the same repetition frequencies asthe sinusoidal waves described above can be used. Also, the lowestfrequency can be one with a period equal to an entire sampling periodinstead of half of a sampling period, or it can have some otherrelationship to the sampling period.

FIG. 2 shows the waveforms of a signal 82 to be sampled, which issampled during periods T which are spaced apart. The multiplying signalsfrom the cosine and sine generators during one sampling period are shownat 84, 85, 86, and 87, for the second through fourth sinusoidalgenerators. It should be noted that the period T does not include pointsat the very beginning or end of the interval, when the commutator outputmay include a rapidly changing signal due to switching to a new channel.

In some applications, every coefficient generator of the samplingcomplex is sampled during each sampling period to transmitallcoefficients A,, through B,,. However, the types of phenomena to bemeasured, such as a temperature or radiation level, generally will nothave rapid variations during a sampling period, and only the first fewFourier coefficients will have an appreciable value. In fact, in manytypical applications, only one of the many phenomena to be sampled islikely to be varying at a rapid rate at any given time. Further savingin bandwidth requirements can be realized by sampling all or most of theFourier coefficient values of a sample only when the higher ordercoefficients are varying rapidly.

FIG. 3 is a block diagram of a circuit for use in connection with thecircuit of FIG. 1 to limit the number of coefficients sampled to onlythose which have a large value. The circuit of FIG. 3 includes a firstcommutator switch 81, a sampling complex 83, and a second commutatorswitch 89, similar to those in FIG. 1. However, the circuit includes acommutator operating circuit 91 for operating a contact 93 in adifferent manner. The operating circuit 91 does not move the contact 93to always contact every input A through B,, before returning to theinitial position A,,. Instead, the circuit 91 is controlled by acompression logic circuit so that the contact 93 returns to its initialposition A at an earlier time, if the higher Fourier coefficients aresubstantially zero.

The compression logic circuit 90 makes the contact 93 return to theinitial position A,, when the contact reaches the last Fouriercoefficient of any appreciable value. At this point, any furthermovement ofcontact 93 would provide additional outputs which were allzero. The logic circuit 90 has inputs 92 which are the outputs from allof the coefficient generators of the sampling complex 83 except thefirst. Thus, the integral values representing the coefficients A,through the highest coefficient, which is labeled B,, in this particularcase, is delivered to the compression logic circuit. If any of thecoefficients A, through 13, has an appreciable value, the OR gate 94will deliver an appreciable output on its output contact 96. If any ofthe coefficient values A, through B is appreciable, then OR gate 98 willdeliver an output at 100. Similarly, if any of the coefficients A,through E, is appreciable, or gate 102 will deliver an output to contact104, while an appreciable coefficient on any of the lines A, through B,will cause OR gate 106 to deliver an output at 108.

When contact 93 is at a first contact A a commutator 114 of thecompression logic circuit is contacting terminal 96. If the voltage at96 is high, both commutator operating circuits 91 and 118 cause theircontacts 93 and 114 to step to the next position, so that contact 93contacts A while 114 contacts 96'. The output from the contact 93 isdelivered to ADC circuit and from there to a buffer storage 122, foreach step of the commutator contact 93.

Contact 93 then steps to B, while contact 114 steps to 100. If theterminal has a low output, indicating that the A; through 8,coefficients are all zero, then a zero signal at 116 is generated. Bothcommutator operating circuits 91 and 118 will then immediately go backto their initial operating positions at A,, and 96, respectively, afterthe sample at B, is taken. No further advances of the contacts 93 and114 occur until the next sampling period. If all of the coefficients areappreciable, however, then both commutator contacts 93 and 114 will stepto the very last contacts B and 112. The contact 112 has a zero valueand causes the return of both commutator contacts to zero. At the end ofthe sampling period, a marker signal generator 120 generates a markersignal which is delivered to the buffer storage 122, to indicate thatthe last sample delivered through the ADC 95 to the buffer storage wasthe last coefficient taken during that sampling period.

The buffer storage 122, which receives the samples and marker signals,may receive one or a large number of samples during each samplingperiod. However, it stores all samples and delivers them at a constantrate. The average rate at which signals are delivered from the bufferstorage 122 to the transmitter 97 is relatively low, as it represents anaverage rate of only a few samples per sampling period T. However, anyone of the channels may have a sample represented by a large number ofcoefficient values taken during the sampling period, during those timeswhen the channel being sampled has a rapidly varying signal. Thus, eventhough detailed information can be transmitted about any of the sampledconditions, the bandwidth requirements for the transmitter 97 arerelatively low.

Provisions can be made in the circuit of FIG. 3 for those situationswhen more than one or a few channels being sampled has rapid variations.Such provisions may take the form of apparatus for raising thecoefficient levels required to turn on the first OR gates 130 through134. Another method is to make the commutator switches return to zeroafter traversing only a limited number of contacts, such as one-half ofthem, regardless of the information present, when the buffer storage 122starts to fill up.

it should be understood that the generation of coefficients and otherprocessing can be performed in either the digital or analogue domain.FIG. 4 illustrates a largely digital circuit which utilizes a commutatorswitch 140 whose output is multiplied by signals of various frequenciesin multiplier circuits 141, 142, 143, and 144, etc., in the same manneras the circuits of H65. 1 and 3v However, the outputs of the multiplyingcircuits are passed to voltage-to-frequency converters 154, 155, 156,157, and 158. The converters generate waves of a high frequency whichare proportional to the voltage inputs thereto. The cycles of theoutputs from the converters are counted in counters 160, 161, 162, 163,and 164. The digital outputs of these counters are entered in a register166 for further processing and delivery to a transmitter 168. Thecounters and register of FIG. 4 are reset at the beginning of each newsampling period.

Although particular embodiments of the invention have been described andillustrated herein, it is recognized that modifications and variationsmay readily occur to those skilled in the art, and, consequently, it isintended that the claims be interpreted to cover such modifications andequivalents.

lclaim:

1. Signal processing apparatus useful in a system which is employed forthe time multiplex transmission of samples from a plurality of signalsources comprising:

commutator means coupled to said plurality of signal sources forsampling said sources during predetermined sampling periods; and

a plurality of sampling means coupled to said commutator means forrespectively providing output signal components, each componentrepresentative of the amplitude ofa different predetermined frequencycomponent in the signal coupled to said plurality of sampling meansduring one of said sampling periods wherein the predetermined frequencyofeach of said components is an integral multiple of a frequency havinga half-period equal to one of said sampling periods, whereby to enable aFourier reconstruction of the sample.

2. Signal processing apparatus as defined in claim 1 including:

an initial sampling means coupled to said input signal for deriving theintegral of said input signal during said sampling period; and whereineach of said plurality of sampling means comprises means for generatinga signal of predetermined frequency, means for multiplying said inputsignal wherein said signal of predetermined frequency, and means forintegrating the product of said input signal and predetermined frequencysignal over a period substantially equal to one of said samplingperiods.

3. Signal processing apparatus as defined in claim 1 wherein:

each of said plurality of sampling means comprises means for multiplyingsaid input signal by a generated signal which is of a frequency that isan integral multiple of a fundamental frequency signal, and means forintegrating said product during a sampling period; and

said generated signals of a first group of said sampling means are atthe same phase as said fundamental frequency signal at predeterminedtimes spaced at integral multiples of said fundamental frequency signal,and said generated signals of a second group of said sampling means areout of phase with said fundamental frequency signal at saidpredetermined times.

4. Signal processing apparatus for sampling a plurality of analoguesignal sources comprising:

commutator means having a plurality of input terminals for coupling tosaid signal sources, an output terminal, coupling means for coupling oneof said input terminals to said output terminal, and means for advancingsaid coupling means to succeeding input terminals at the end ofpredetermined sampling periods; and

sampling complex means coupled to said output terminal,

including a plurality of coefficient generators for generating Fouriercoefficients representing the amplitude of components of predeterminedfrequencies in inputs to said sampling complex during individualsampling periods.

5. Signal processing apparatus as defined in claim 4 wherein:

a plurality of said coefficient generators comprises means forgenerating a signal of predetermined frequency, means for multiplyingthe output of said commutator means by said signal of predeterminedfrequency, and means for integrating the product of said means formultiplying over a period substantially equal to one of said samplingperiods.

6. Signal processing apparatus as defined in claim 4 including:

means responsive to the amplitude of said Fourier coefficients fortransmitting signals representing only coefficients of lower frequencycomponents when coefficients of higher frequency components are lessthan a predeter mined level during a sampling period.

7. Signal processing apparatus as defined in claim 4 including:

means responsive to the amplitude of said Fourier coefficients forgenerating output signals representing the amplitude of only groups ofcoefficients containing coefficients of more than a predeterminedamplitude while generating no output signals for groups of coefficientscontaining no coefficients of more than said predetermined amplitude,for coefficients taken during a sampling period; and

buffer storage means responsive to said output signals for transmittingsignals representing said Fourier coefficients at a substantiallyconstant rate, whereby to utilize a lower maximum bandwidth.

